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Consider reducing the aperture to an absurd level and think of a pinhole camera which has a ridiculously high f-number and no (curved) glass at all. DoF is about as big as it gets. The more you increase the aperture then, to stop the image blur increasing, you have to introduce a lens (curved glass) to bring the scene into focus. The bigger the aperture the stronger (more curved) that lens has to be, everything else being equal. The stronger the lens the tighter the range of distances in the scene being photographed which that lens can bring into sharp focus becomes. If that concept is difficult then think how easily the human eye can compensate for an error in focus when looking through a weak spectacle lens held at arm's length compared to looking at the same scene through a magnifying glass held at arm's length.

These two "pictures" where taken at the same focal length at the same distance from a subject. The black vertical lines indicate the aperture opening. The red lines are sight lines from the top and bottom of the aperture opening. As you can see, they approach the subject at a shallower angle. When they meet (or get close to meeting) - they show a focused subject.

In the bottom graphic they are closer longer - thus, a longer depth of field.

Now, I know what your thinking - they only meet at one point in both situations. This is true. There will only be one point in the photograph that will be exactly in focus. However, your brain has a tolerance.

If you notice the lines are closer together in the bottom figure than they are in the top for a longer time. Your brain interprets "close to being in focus" as "being in focus"... again, with a tolerance. Your eyes just don't have the precision to detect see the minor loss of focus - so it still looks in focus.

Found an answer... it's actually true. Your eye's DoF is more shallow in the night. This is why people with bad vision (i.e. inability to focus well) have more trouble during seeing during the night. The DoF is shallower and thus your eye's "autofocus" demands more precision. Huh.